The present invention relates to a structure of a metal halide lamp for an automobile headlight.
In recent years, as a lamp for an automobile headlight, a new small metal halide lamp that can be substituted for a conventional tungsten halogen lamp has been developed and commercially expanded. This metal halide lamp has the advantage that it can achieve a luminous flux three times as high as that of the conventional tungsten halogen lamp while the 35 W lamp power of the metal halide lamp is smaller than the 55 W lamp power of the conventional tungsten halogen lamp. On this account, the spread of this metal halide lamp has been promoted as a next-generation lamp capable of achieving improved brightness as well as energy saving and allowing still safer drive of an automobile, particularly at night.
FIG. 9 shows a structure of an arc tube of such a metal halide lamp for an automobile headlight. An arc tube 19 of the lamp has the structure as follows. An envelope 20 of the arc tube is made of quartz, and electrodes 21 and 22 made of a pair of tungsten bars are provided at both ends of the arc tube. Molybdenum foils 25 and 26 are sealed hermetically in sealing end parts 23 and 24 of the envelope 20, and a rear end of the tungsten electrode 21 is welded and connected to one end of the molybdenum foil 25 and a rear end of the tungsten electrode 22 is welded and connected to one end of the molybdenum foil 26. External leads 27 and 28 are welded and connected to the other ends of the molybdenum foils 25 and 26, respectively. Inside the arc tube, 0.01 to 1.0 mg of a mixture of scandium iodide and sodium iodide (NaI+ScI3) is sealed as a main component of a luminescent material 29 together with 0.1 to 1.0 mg of mercury and 0.1 to 1.5 MPa of xenon as buffer gases 30. Typically, the size of the arc tube 19 is such that the distance Le between the electrodes is 4.2 mm, the inner diameter xcfx86i of the arc tube is 2.8 mm, and the inner volume of the arc tube is 30 mm3 at maximum. It is to be noted here that other metal halide materials such as ThI4, LiI, TlI, and the like also may be sealed in the arc tube.
The lighting operation of the above-mentioned metal halide lamp for an automobile headlight is different from that of a normal metal halide lamp for general lighting. Specifically, during the lighting operation of the above-mentioned metal halide lamp, flickering occurs repeatedly, including the flickering at the time of a so-called xe2x80x9cinstantaneous restartxe2x80x9d. Besides, in order to obtain the required luminous flux immediately after the lamp is turned on, the lamp immediately after being turned on is subjected to a lamp current of 2.6 A, which is about seven times as high as the lamp current of 0.4 A during the steady-state lighting. As described above, the metal halide lamp for an automobile headlight is operated according to a unique, relatively demanding lighting system.
At the beginning of the development of the conventional metal halide lamp shown in FIG. 9, a first problem was found that the quartz present in the sealing end parts 23 and 24 of the envelope 20 of the arc tube 19, especially around portions of the tungsten electrodes 21 and 22 sealed therein (hereinafter, such portions are referred to as xe2x80x9csealed portionsxe2x80x9d), may have cracks and/or may be damaged within a relatively short time of 500 hours or less, resulting in a short lifetime of the lamp. It can be said that this problem is related to the unique lighting system as described above.
Since then, various studies have been made to solve the above-mentioned problem, and means for solving the problem are disclosed, for example, in JP 7(1995)-282719 A, JP 7(1995)-21981 A, JP 10(1998)-223175 A, JP 10(1998)-269941 A, etc.
As a means for preventing the occurrence of cracks and/or damage in the quartz present around the sealed portions of the electrodes, electrodes 21 and 22 made of a so-called xe2x80x9cthoriated tungsten materialxe2x80x9d that contains thorium oxide (ThO2) as an additive particularly in an amount of 1 to 2 wt % has been employed. By using such electrodes, the adhesive strength between the electrodes and the quartz can be increased. Further, as shown in an enlarged view of FIG. 10 illustrating the sealing end part 23 of the envelope, a quartz coating 31, which is not mechanically connected to the quartz present in the sealing end part 23, is formed on the periphery of the tungsten electrode 21 (The same occurs on the periphery of the tungsten electrode 22).
As a result, the quartz present in the sealing end part 23 is no longer subject to stress distortion due to the difference in thermal expansion between the tungsten electrode 21 and the quartz at the time of the flickering of the arc tube 19. Cracks and/or damage occurring in the quartz present in the sealing end part 23 thus can be prevented. Since this means is extremely effective in preventing the occurrence of cracks and/or damage in the quartz, it has been a major technique generally applied to the conventional lamp shown in FIG. 9.
Another effective means is winding a tungsten coil 32 on the periphery of the sealed portion of the tungsten electrode 21 of the arc tube 19 as shown in FIG. 11. In contrast to the case where the above-mentioned means using the thoriated tungsten electrodes is employed, the adhesion between the tungsten electrode 21 in the sealing end part 23 of the envelope and the quartz on the periphery of the electrode 21 is made very weak by using the tungsten coil 32. However, in this case, the quartz on the periphery of the electrode still is subjected to less stress distortion at the time of the flickering of the arc tube, thereby allowing cracks and/or damage occurring in the quartz to be prevented. It is to be noted that similar means have been applied to a conventional tungsten halogen lamp.
A second problem found in the development of the conventional metal halide lamp shown in FIG. 9 is that the lumen maintenance factor of the arc tube 19 decreases with the passage of time after the lamp is turned on. As described above, the arc tube 19 having a small volume is operated at a lamp power of 35 W in the steady state. In addition, the arc tube 19 is operated according to a demanding lighting system in which instantaneous restart, high-current operation immediately after the lamp is turned on, etc. are required. Thus, the operating temperature of the envelope 20 made of quartz rises particularly remarkably to reach 1000xc2x0 C. or more, and the tungsten electrodes 21 and 22 are evaporated and/or worn away considerably, which cause devitrification and blackening of the envelope 20. Therefore, the decrease in lumen maintenance factor of the lamp due to the devitrification and blackening of the envelope 20 cannot be avoided.
As disclosed in JP 7(1995)-21981 A, for example, the thoriated tungsten electrodes 21 and 22 containing ThO2 as a means for solving the first problem has been regarded as effective also in solving the second problem and employed as a means for solving the second problem. Conventionally, there has been a widely accepted theory as follows in the art. That is, in a high-pressure discharge lamp employing the thoriated tungsten electrodes, a monatomic layer of Th is formed during the lamp operation to decrease the work function at front ends of the tungsten electrodes and thus decrease the operating temperature thereof considerably, thereby suppressing the blackening of the arc tube caused mainly by the tungsten evaporated from the tungsten electrodes. In fact, in a conventional NaIxe2x80x94ScI3-based metal halide lamp for general lighting, thoriated tungsten electrodes generally have been used to suppress a decrease in the lumen maintenance factor caused by the blackening of the arc tube during the lifetime of the lamp.
As a specific means for solving the second problem, a manufacturing process has been developed and introduced into service that provides a high-purity lamp by sufficiently removing impurities such as H2O contained in a sealed luminescent material and in quartz forming an envelope as well as impurities such as H2O and O2 that entered during the manufacturing process. According to this process, the conventional metal halide lamp shown in FIG. 9 achieved the required average lumen maintenance factor of 70% during the lifetime of 1500 hours, which is the required lifetime in the early stages of its development. It is to be noted that the lumen maintenance factor is determined as a percentage of the luminous flux after 15-hour aging to an initial luminous flux.
In recent years, the metal halide lamp for an automobile headlight is facing a new task of extending its lifetime. The lifetime of 1500 hours at the early stages of its development is determined considering the cracks and/or damage occurring mainly in the quartz present in the sealing end parts of the envelope of the arc tube. However, since this problem was solved by the above-mentioned means, there has been a demand from users that the lifetime of 1500 hours required in the early stages of its development should be extended to 2000 hours or more. The lifetime of 2000 hours or more corresponds to an average travel distance of about 100,000 km or more of an automobile and thus allows the lamp to be treated as a substantially maintenance-free component. This provides a great advantage to the users.
According to an actual life test for 1500 hours or more conducted with respect to the conventional metal halide lamp, it has been revealed that, in addition to the problem (a) that the average lumen maintenance factor decreases to be below the required value of 70%, the following new problems (b)-(d) occur remarkably during the lifetime of 1500 hours or more for the metal halide lamp: (b) color temperature and/or chromaticity coordinates are changed; (c) the luminance at the center of the arc (hereinafter, referred to as xe2x80x9carc center luminancexe2x80x9d) may be decreased due to the diffusion of an arc discharge region in the arc tube oriented so as to extend horizontally during lighting; and (d) the arc tube is expanded (the inner diameter of the envelope is increased) by the heat generated during lighting.
As described above, in order to realize a long-life metal halide lamp for an automobile headlight having a lifetime of 2000 hours or more in response to the users"" demand, a major technical task specifically is improving the average lumen maintenance factor so as to achieve the required value of 70% or more during such a long lifetime of the lamp as well as further improving other life characteristics of the lamp.
The present invention is intended to solve the above-mentioned problems in the prior art. It is an object of the present invention to provide a long-life metal halide lamp for an automobile headlight that can achieve a further improved lumen maintenance factor and other life characteristics during 2000 hours or more of lighting.
In order to achieve the above-mentioned object, a metal halide lamp for an automobile headlight according to the present invention includes an arc tube in which a pair of tungsten electrodes are provided at both ends and a metal halide as a main component of a luminescent material and xenon gas as a buffer gas are sealed, wherein the tungsten electrodes contain not more than 0.4 wt % of thorium oxide, and the metal halide contains scandium iodide.
This makes it possible to achieve the required average lumen maintenance factor of 70% or more during the lifetime of 2000 hours. Besides, changes in color temperature, expansion of the arc tube due to the heat generated during lighting, and a decrease in the arc center luminance can be suppressed. As a result, a long-life lamp having a lifetime of 2000 hours or more can be provided to satisfy the users"" demand.
Further, in the metal halide lamp for an automobile headlight according to the present invention, a pressure of the xenon sealed in the arc tube is at least 0.4 MPa.
This makes it possible to suppress the blackening of the arc tube caused by evaporation and/or wear of the tungsten electrodes in spite of the fact that the tungsten electrodes in the lamp according to the present invention contain a smaller amount of thorium oxide as compared with those in the conventional lamp. Thus, the lamp can achieve the required average lumen maintenance factor of 70% or more during the lifetime of 2000 hours. As a result, a long-life lamp having a lifetime of 2000 hours or more can be provided to satisfy the users"" demand.
Further, considering the pressure resistance of the arc tube, it is preferable that the pressure of the xenon sealed in the arc tube is not more than 1.0 MPa. Of course, the upper limit of the pressure varies depending on the thickness and the like of the arc tube.
Furthermore, in the metal halide lamp for an automobile headlight according to the present invention, a lamp current at least three times higher than that applied during steady-state lighting is applied to the metal halide lamp during a period after the lamp is turned on and until the steady-state lighting is established.
Still further, in the metal halide lamp for an automobile headlight according to the present invention, it is preferable that the following relationship is satisfied:
({fraction (1/12)}xc3x97me+mx)/Vxe2x89xa75(mg/mm3)
where V (mm3) represents an inner volume of the arc tube, me (mg) represents a total weight of thorium elements contained in portions of the tungsten electrodes protruding into a hollow space inside the arc tube, and mx (mg) represents a total weight of thorium elements present in the hollow space except for those contained in the portions of the tungsten electrodes protruding into the hollow space.
This makes it possible to prevent the cracks and/or damage liable to occur in the quartz present in the vicinity of the electrode-side ends of the molybdenum foils in the sealing end parts of the envelope of the arc tube during 2000 hours or more of lighting. As a result, a long-life lamp having a lifetime of 2000 hours or more can be provided to satisfy the users"" demand.
Still further, in the metal halide lamp for an automobile headlight according to the present invention, it is preferable that the tungsten electrodes contain not more than 0.2 wt % of thorium oxide.
This makes it possible to further improve a lumen maintenance factor and other various life characteristics during 2000 hours or more of lighting. As a result, a long-life lamp having a lifetime of 2000 hours or more and also a still higher quality can be provided to satisfy the users"" demand.
Still further, in the metal halide lamp for an automobile headlight according to the present invention, it is preferable that the metal halide contains sodium iodide.
Still further, in the metal halide lamp for an automobile headlight according to the present invention, it is preferable that, between each of the tungsten electrodes and the arc tube, a buffer member for reducing stress distortion due to a difference in thermal expansion between each of the tungsten electrodes and the arc tube is provided.
Still further, in the metal halide lamp for an automobile headlight according to the present invention, it is preferable that the buffer member is an intermediate sealing glass having a thermal expansion coefficient smaller than that of tungsten and greater than that of quartz.
Still further, in the metal halide lamp for an automobile headlight according to the present invention, it is preferable that the buffer member is a tungsten coil wound around a portion of each of the tungsten electrodes that is sealed in a sealing end part of an envelope.
This makes it possible to prevent the occurrence of cracks and/or damage in the quartz present around the tungsten electrodes in the sealing end parts of the envelope when the lamp is turned on or turned off in spite of the fact that the tungsten electrodes in the lamp according to the present invention contain a smaller amount of thorium oxide as compared with those in the conventional lamp. As a result, a long-life lamp having a lifetime of 2000 hours or more can be provided to satisfy the users"" demand.
Still further, in the metal halide lamp for an automobile headlight according to the present invention, it is preferable that a portion of each of the tungsten electrodes that is sealed in a sealing end part of an envelope is coated with at least one material selected from the group consisting of rhenium, platinum, rhodium, ruthenium, and gold.
This makes it possible to prevent the occurrence of the cracks and/or damage in the quartz present around the tungsten electrodes in the sealing end parts of the envelope when the lamp is turned on or turned off in spite of the fact that the tungsten electrodes in the lamp according to the present invention contain a smaller amount of thorium oxide as compared with those in the conventional lamp. As a result, a long-life lamp having a lifetime of 2000 hours or more can be provided to satisfy the users"" demand.
Still further, in the metal halide lamp for an automobile headlight according to the present invention, it is preferable that a metal foil is sealed in each sealing end part of an envelope of the arc tube, and each of the tungsten electrodes is connected electrically to the metal foil via a buffer member for reducing stress distortion due to a difference in thermal expansion between each of the tungsten electrode and the arc tube.
Still further, in the metal halide lamp for an automobile headlight according to the present invention, it is preferable that the buffer member is a conductive member having a thermal expansion coefficient smaller than that of tungsten and greater than that of quartz.